Radiological evaluation of breasts is important not only for early detection of disease in breasts, but also for staging, treatment targeting and monitoring responses to treatment. Conventional x-ray mammography has been shown a cost-effective tool for early detection of breast cancer. In conventional x-ray mammography, a breast is positioned on a platform and compressed with plastic plates. The breast is compressed in order to even out the breast thickness so that the correct dose of X-rays can be delivered for clear images without over exposure in thinner regions and under exposure in thicker regions and also to spread out the tissue to reduce the likelihood of obscuring benign and malignant lesions at different tissue levels. Two images of the breast, medio-lateral oblique (MLO) and cranio-caudal (CC), are typically made at an oblique angle to each other to increase the likelihood of seeing features in the breast that are not recognizable from one direction, but which may be discemable in another direction.
The predictive value and specificity of x-ray mammography remain limited however, due to projecting a three-dimensional object into a two-dimensional image and due to poor contrast detectability. Incorrectly diagnosing malignant tissue as healthy can result in missing cancers in their early stages while incorrectly diagnosing healthy tissue as malignant can lead to unnecessary surgical procedures. In indeterminate cases, biopsy is often necessary, despite the disadvantages of high cost and the stress imposed on patients.
Cone beam CT systems have been used to provide three-dimensional images of uncompressed breasts of patients. In cone beam CT, a patient lies in a prone position on a couch having a hole in it. The patient's breast extends down through this hole and a cone beam CT machine rotates around it about a vertical axis of rotation. This system has minimal radiation dose to the rest of the patient's body (including the heart and lungs) since only the breast itself is exposed to the radiation beam. One limitation of this type of system is that it only images the breast itself without providing image information on the important soft tissue between the breast and the axilla (arm pit) where primary breast lesions often spread to infect lymph nodes in this region. Another limitation is that it cannot make immediate use of widely available CT gantry systems and external beam radiotherapy systems, which primarily have axes of rotation that are horizontal.
Spiral CT scanners have been used to produce images of breasts and surrounding tissues. In spiral CT, a patient lies on a couch, which is moved into a gantry where a radiation source rotates many revolutions about a horizontal axis of rotation to produce the spiral. Since the radiation source is always pointed directly at the patient as it moves around, all the tissue of the chest adjacent to the breasts is exposed to the radiation beam from multiple directions to produce continuous flow of images in relative short period of time. One limitation of this type of system is that it scans not only the target breast and surrounding soft tissue, but also simultaneously exposes the heart, lungs and other body parts to unnecessary radiation.
Accordingly, there is a need for an improved system and method for computed tomography of breasts and radiotherapy of tumorous tissues in breasts.